90 degree optical hybrids are widely used within coherent optical receivers to mix four in-phase and quadrature states (B; −B; jB; −jB) of a reference optical signal (B) with a signal to be detected (A). 90 degree optical hybrids are six-port optical devices which generally comprise a pair of linear dividers and a pair of optical signal combiners interconnected by four waveguides, one of which includes a 90 degree phase shifter. In a coherent optical receiver the four vectorial additions of the reference signal and the signal to be detected are detected at two pairs of balanced photodetectors. Various designs for a 90 degree optical hybrid are proposed in Seimetz et al, “Options, feasibility and availability of 2×4 90° hybrids for coherent optical systems”, Journal of Lightwave Technology, volume 24, number 3, pp. 1317-1322, March 2006. The prior art designs of 90 degree optical hybrids suffer the problem of cross-talk between waveguides, which can strongly degrade the performance of a coherent optical receiver. Prior art 90 degree optical hybrids can also suffer the problem of an additional phase-shift being introduced due to external factors to one or more signals during propagation through the respective waveguides. This can lead to an imprecise phase shift within the 90 degree optical hybrid, and thus an imprecise phase difference between optical signals to be mixed, resulting in degrading of the in-phase and quadrature signals and a lack of symmetry between the two balanced detection processes of a coherent optical receiver.